With the spread of the Internet, communications have become part of infrastructures in today's society. An amount of data dealt with per user is increasing yearly and so is a network traffic. In an optical fiber transmission line that particularly takes a major role in trunk communications among network infrastructures, an optical transmission system with a capacity of 100 Gb/s (Giga bits per second) per channel has been commercialized. In the days ahead, extending the range of 100-Gb/s optical transmission system and increasing the capacity of a 400 Gb/s optical transmission system and the like are anticipated.
In a 100-Gb/s optical transmission system, Quadrature Phase Shift Keying (QPSK) scheme is used as a modulation scheme. A polarization-multiplexed QPSK signal is received with a combination of a coherent optical front-end and a digital signal processor.
A typical configuration of a digital coherent optical receiver is illustrated in FIG. 9 (see Patent Literature 1, for example). A related digital coherent optical receiver 600 includes an optical front-end 610, an analog-digital (A/D) converter 620, and a signal processing LSI 630. The signal processing LSI 630 includes a dispersion compensation unit 631, a polarization splitting unit 632, and a carrier recovery unit 633.
In the optical front-end 610, polarization-multiplexed QPSK modulated signal light is converted into electrical signals XI, XQ, YI, and YQ that are orthogonal components in polarization and phase. These electrical signals are converted into digital signals by the A/D converter 620 and then are demodulated in the signal processing LSI 630. The dispersion compensation unit 631 included in the signal processing LSI 630 compensates for waveform distortion caused by the chromatic dispersion that a QPSK signal has suffered while propagating through an optical fiber transmission line. The polarization splitting unit 632 separates a polarization-multiplexed signal by controlling filter coefficients of an adaptive equalization filter using an algorithm such as the Constant Modulus Algorithm (CMA). The carrier recovery unit 633 compensates for a phase difference and a frequency difference between signal light and local light and recovers a carrier signal.
The dispersion compensation unit 631 requires a filtering operation circuit with several hundreds to several thousands of taps in order to compensate for dispersion that amounts to several thousands to several hundreds of thousands picoseconds per nanometer (ps/nm). On the other hand, a range of variation with time in the chromatic dispersion that occurs in an optical transmission line is small. For these reasons, the dispersion compensation unit 631 is implemented by a Frequency Domain Equalization (FDE) circuit, which has high circuit efficiency because of a large-scale filtering operation although the control of the filter coefficients is fixed.
The polarization splitting unit 632 requires an adaptive equalization circuit that is capable of adaptively controlling a filter coefficient in order to follow polarization variations of several kHz. Because the polarization mode dispersion that occurs in an optical transmission line is about several tens of picoseconds, a filtering operation circuit with a small number of taps can be used sufficiently. For these reasons, the polarization splitting unit 632 is implemented by a Time Domain Equalization (TDE) circuit, which is capable of adaptively controlling the filter coefficients.